The iron oxide surfaces are excellent catalysts for ammonia production, known as the Haber-Bosch process. At equilibrium conditions, hydrogen (H2) and nitrogen (N2) react on an iron surface to produce ammonia, where adsorption and dissociation of N2 is the limiting step in the reaction. Although the ammonia production occurs at high temperatures and pressures under equilibrium conditions, there is a gap in knowledge of how the catalyst surface transforms in the near-ambient pressure (intermediate) regime, between ultra-high vacuum (surface science) with (industrial) reaction conditions. We present recent results of ambient pressure-X-ray photoelectron spectroscopy (AP-XPS) that measure changes in the surface structure and oxidation states under near-ambient pressure conditions. AP-XPS is complemented with polarized modulated-infrared reflection absorption spectroscopy (PM-IRRAS) to measure the adsorption of N2, H2, and O2 on single crystal Fe3O4(001) and α-Fe2O3(0001) surfaces to gain a better understanding of the fundamental surface reactions associated with ammonia formation. Additionally, the structural changes from surface oxidation and reduction of single crystal Fe3O4(001) and α-Fe2O3(0001) surfaces were observed in the valence band region and show how the crystal structure of magnetite and hematite are transformed to different phases. These studies demonstrate how complex surface chemistry in a reactive gas atmosphere restructures the top layers of iron oxide surfaces. Despite that these experiments are far from equilibrium conditions, the results address fundamental questions of iron oxide surfaces regarding the oxidation states and binding energies of N2. These studies bridge the gap between previous ultra-high vacuum studies and reaction conditions to provide insight into other nitrogen-based chemistries on iron oxide surfaces that affect our agriculture and energy industries.

Understanding adsorption processes on iron oxide surfaces for ammonia formation using ambient pressure-XPS

256th National Meeting (2018)

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